Thank you so much for cluing me in to how that rule applies when the device is off too. Wow. In modular synthesis its fairly common practice to leave things patched together to resume later, and one way or another power cycles do happen, so that seems critical.
Here's what looks good so far. The black background is new circuitry while the white is the synth's existing. It really is nice to have the 5V supply right there.
In simulation when I adjust the source voltage to sweep above 5V and below 0V, it clamps them right off. Sweeeeet. So maybe front panel trimmer in addition to the 1k input resistor would be helpful for pulling down signals that are getting clipped.
Current is going to vary from these external sources I'm afraid. All I can really say off-hand is that with the Microbrute LFO/Envelope, there is enough to use a passive splitter Y cable to modulate more than one thing with the same source and not lose much voltage swing. The current I get from the computer CVs with the AC encoding plugin suite are probably considerably lower, being from line level audio signals put through voltage multiplier circuitry. And everything has different impedances of course. C'est la vie in creative audio work.
Anyway, this brings me back to the switching.
The simplest procedure I see for switching between the joystick and the inputs (potentially having one axis controlled externally and the other internally) is to put the two signals through a SPDT like in the red below:
This makes the most sense to me because it doesn't leave the potentiometers draining the 5V source with both pins 2 and 3 shorted to ground, especially when I will now depend on that 5V source for the schottky protection circuit.
The ideal here would be to use 1/8" jacks/sockets with a built-in normally closed SPDT, routing the joystick to CPU when unplugged. But switching jacks turn out to be a less flexible than I'd hoped and most just short the "extra" signal (e.g., device power) to sleeve/ground when unplugged, which is suitable for most audio applications. 3-connector TRS (stereo headphone style) jacks will be acceptable because the ring connector just goes to ground. So that increases my options, but I'm still not finding anything that looks quite right even when I look at schematics/datasheets. I'm seeing things like this and it doesn't seem like what I want:
Am I just going about this all wrong? Would it maybe be okay to short V-X and V-Y from the potentiometers to ground when an external source is plugged in, and still count on enough of the 5V signal to the schottky diode (and maybe other parts of the device)? I realize that might be impossible to answer when we don't know how much current is available from the +5V source. All I can get from the service manual is this, with separate +5V supplies for analog and digital circuits. This isn't the only thing using that same +5VA of course, but I figured that would still be shorting a lot of it to ground.
I appreciate all the help and I know I'm asking a lot here while still not showing a good grasp of some basics. I'm doing my best to learn what I can on my own and be clear and thorough, but your time is still your time, so thanks.